Power amplifiers (PAs) are used in wireless devices to amplify a radio frequency (RF) signal to a desired output power level for transmission from the wireless device. Different wireless communication systems have different requirements for output power levels. In some systems, the dictated power levels may vary from low power levels to high power levels, depending on a type of communication that is occurring. For example, different communication systems such as enhanced data rates for GSM evolution (EDGE), long term evolution (LTE/4G), WiFi in accordance with an IEEE 802.11 standard, worldwide interoperability for microwave access (WiMax), code division multiple access (CDMA), and wideband-code division multiple access (W-CDMA), all have different power requests. In certain systems, the portable transmitter may have two or more power modes wherein each mode has a different maximum output power capability. For example, a transmitter may have a high power mode with a maximum output power of 27 dBm, a medium power mode with a maximum of 17 dBm and a low power mode with a maximum of 7 dBm. With such a system, energy used from the battery can be conserved when lower powers are transmitted.
In typical wireless implementations, an output load presented to the PA is typically 50 ohms. The relationship between the output power, load, and voltage switching may be determined in accordance with the following equations:
      P    out    =            V              o        ,        rms            2              R      L      (where Pout is output power, Vo, rms and Vo, pk are output voltage RMS and peak values, and RL is load resistance).
In turn, the efficiency, η, of a PA, which is desirably as high as possible, is derived as the ratio of output power to the power consumed from the supply voltage:
      η    =                  P        out                    P        sup              ,where Psup=Vsup·Isup, where Isup is the current drawn from a supply (such as a battery) and may be supplied to a regulator device, and Vsup is the supply voltage.
If any voltage (ΔV) is dropped across a linear regulator, also known as a Low Dropout Regulator (LDO), it results in power dissipated (or lost) in the regulator. Such losses directly impact the total PA efficiency. Thus, in the case of LDOs, for a high efficiency implementation, the output voltage of the LDO (which is the Vdd for the PA circuits) should be maximized with as little voltage dropped across the regulator as possible. For a given load, this however determines the maximum power that can be delivered to the load. To lower output power (Pout), the input signal level can be reduced (a.k.a. power backoff) at the cost of much reduced efficiency. To efficiently control the Pout delivered to the load, the Vdd (or Vpa) needs to be lowered. This backoff in power, if accomplished with a linear regulator, will result in a large voltage drop (ΔV), resulting in low PA efficiency.
Accordingly, to attempt to maintain efficiency, most PAs adjust the supply voltage level by means of a switching regulator which is used to supply the variable voltage level for the desired Pout levels. The key characteristic of a switching regulator (also known as a DC/DC regulator) is that it transforms the supply (or battery) voltage to a different voltage (e.g., Vdd for the PA) with minimal power loss. Because of the characteristics of such a DC/DC switching regulator, large off-chip components such as an inductor are needed, as significant current needs to be supplied to the PA for the high power mode. Furthermore, because of the switching frequency of the switching regulator, large spurs may exist, at the harmonics of the switching frequency that can cause noise at the PA output. Still further, there is always an efficiency impact due to the DC/DC converter, and a more complex feedback mechanism or other control (such as factory calibrations) that are needed to properly control the output voltage of the switching regulator for a desired output power.